This invention relates to a fuel supply control system for use with an internal combustion engine, and more particularly to such system in which a fuel quantity adjusting means, which supplies an air/fuel mixture to the engine, is controlled so as to lean the mixture when the engine is operating in a low load condition. The system is therefore enable to achieve curtailment of the fuel consumption without spoiling the driveability and emission characteristics of the engine.
A fuel supply control system is conventionally known which is adapted particularly for use with a gasoline engine and in which a fuel-injection type fuel quantity adjusting means for supplying fuel to the engine has its valve opening period electronically controlled so as to control the air/fuel ratio of an air/fuel mixture being supplied to the engine to a predetermined value.
As one of such fuel supply control systems there has been proposed e.g. by U.S. Pat. No. 3,483,851 a system which is adapted to determine the valve opening period of the fuel quantity adjusting means by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc.
On the other hand, in these days there is a tendency for automobile fuel cost to gradually increase. To cope with this tendency, it has conventionally been employed to lean the mixture being supplied to the engine, that is, set the air fuel ratio of the mixture at a higher value than the stoichiometric value for reduction of the fuel consumption.
While the three-way catalyst, which is conventionally used for purifying unburned hydrocarbons, carbon monoxide and nitrous oxides in the exhaust gases emitted from an engine, shows the best conversion efficiency of such ingredients when the engine is operating in an air/fuel ratio near the stoichiometric air/fuel ratio, it is known that leaning of the mixture reduces the hydrocarbons and carbon monoxide being emitted from the engine and further leaning of the mixture also reduces the nitrous oxides being emitted from the engine. Therefore, the emission characteristics would not be spoiled by leaning the mixture.
However, since leaning of the mixture causes a drop in the output of the engine, the mixture should not be leaned when the engine is operating in a large torque-required condition such as rapid acceleration and wide-open-throttle, to avoid poor driveability. In such large torque-required conditions, the mixture should be enriched so as to obtain a required engine output. For this reason, conventionally, leaning of the mixture is effected when the engine is operating in a low load and steady condition where large torque is not required, such as a cruising condition.
Engine idle operation falls in the category of the low load steady condition of the engine, in which leaning of the mixture is also effected in some engines. However, the operating region of the engine to which the present invention is directed is a mixture leaning region corresponding to higher engine rpm's than usual idling rpm. If leaning of the mixture is continuously carried out when the engine is rapidly accelerated from its idling state, required acceleration performance cannot be obtained, resulting in deteriorated driveability. That is, the mixture leaning operation should be interrupted when the engine is rapidly accelerated into the above higher engine rpm mixture leaning region from its idling state.
For the same reason as above, the mixture leaning region should be discriminated against other large torque-required engine operating regions so that in the latter operating regions the mixture is not leaned.
Further, the engine temperature should also be taken into consideration for effecting the mixture leaning operation. If the mixture is in a lean state when the engine temperature is low, the phenomenon called misfire will occur that no firing takes place in the engine cylinders.
As mentioned above, leaning of the mixture should be made when the engine is operating in a low load condition. Generally, detection of the low load condition of the engine is made by detecting pressure in the intake pipe of the engine. In this case, if the pressure in the intake pipe is detected by means of a gauge pressure sensor, the detected pressure value varies with changes in the ambient atmospheric pressure, which can bring about disadvantages such as the one that in high altitudes leaning of the mixture is not effected even when the engine is operating in such a condition as requires leaning of the mixture. Therefore, to determine the mixture leaning condition with accuracy, the intake pipe pressure needs to be detected by means of a sensor operable without being influenced by ambient atmospheric pressure, i.e., an absolute pressure sensor.
The above-mentioned various requirements must be taken into consideration to carry out leaning of the mixture.